I haven't run a 5e demon lord against my group yet, but they're currently in Vault of the Drow with plans to move onto Q1 after, so a confrontation with Lolth is inevitable. They will be around 18th-20th level by then, so I'll likely use this version of Lolth or a tweaked variation of it.Has anyone fought a demon lord, perhaps one from Mordenkainen's Tome of foes or Out of the Abyss, at full power? I imagine it would take a party of high level, if not max level, players to even survive such a battle, much less triumph.
In my experience, a solo demon lord has never stood much of a chance against a very high level party (15th and up) in any edition of D&D - it's not exclusively a 5e issue. Rather than debate the system or stat block tho, I think it behooves any DM to customize the encounter to properly challenge the PCs. Every party has its strengths and weaknesses, so the DM can use that knowledge to create boss fights that exploit both without the BBEG coming across as omniscient. Its a balancing act that can really challenge a DM's creative output - whether it means adding mooks, terrain, magical effects, just beefing up the boss (demon lord, in this case), or a combination of some or all of those things - but its ultimately more rewarding (and more fun) for everyone at the table.